HELP iMac G5 FREEZING!!!!!!!!!!!

Hey guys i have been having a problem lately with my iMac G5. First of all it is the iMac G5 with the iSight 20" 2.1GHz, 1.5GB ram. The other day when i went to use my computer in the morning it was locked up and the clock had read 1:49 AM and it was 8:30 AM so something is causing it to lock up and the only thing i can do to fix it is shut it down by holding the power button for 4 seconds. Well this just happened again this morning on me except the clock was stuck at 11:45 PM and it was 8:45 AM so i am not sure if any of you guys have had a problem simmilar to this or if you can help as to what you think it might be. Thanks

Doesn't anyone have any ideas?

Anyone have any ideas at all? I tried reseting the PRAM we will see if it happens again.

the only thing i use on my computer at night is front row. i use it to watch a movie or listen to music at night. and yes i repaired permissions just the other day.

Ok i just verified the disk and it sais volume needs repair how do i repair it?
The repair button is grayed out and won't let me use it.

Ok i repaired the drive now i guess it is a wait and see game to see if it happens again. This is a time that i am glad i do a full backup every night of my system.